Contrasting effects of chlorides and sulphates on the hydrogen ion concentration of acid solutions

In the course of an investigation upon the effect of neutral salts upon hydrogen ion concentration we have noted a striking contrast in the effect of chlorides, sulphates and magnesium salts on the apparent hydrogen ion concentration of solutions of hydrochloric and of sulphuric acids.

The results of our measurements are given below. In all cases a solution of acid was mixed with a solution of a given salt and diluted to 100 c.c. The concentration of this solution of the mixture was always 0.1 normal with respect to the acid and that particular concentration of the salt whose effect upon the hydrogen ion concentration was to be studied. The hydrogen ion concentrations were measured two days after the solutions were made up.

The effect of chlorides in increasing the hydrogen ion concentration of hydrochloric acid solutions is not new. This has previously been shown by Harned 1 and by Fales and Nelson 2 . Arrhenius 3 found that the hydrogen ion activities of acetic and of hydrochloric acids were increased by neutral salts which he thought was due to the salts increasing the dissociation of the acids. It is possible that recent work on the hydration of ions in aqueous solution will offer a more plausible explanation.

When the chlorides are arranged in order of their ability to increase the hydrogen ion activity, we get the following series: MgCl₂ > BaC1₂ > LiCl > NaCl > NH₄CI = KC1 which, with the exception of magnesium and barium is in inverse order to a series of these cations arranged in order of their equivalent conductivities.

In the above series the salts are arranged in order of the increasing hydration (i.e., number of molecules of water combined with the ions at infinite dilution) of their cations-K⁺, 9.6; NH4⁺, 10.7; Na⁺, 16.9; Li⁺, 24, ol Ba⁺⁺, relatively higher;² Mg⁺⁺, higher than Ba⁺⁺.³